1. Field of the Invention
The present invention is directed to the papermaking arts, more particularly to a process for the manufacture of a paper having improved grease and water resistance and increased tensile strength, yet facilitating recycling of the paper. Such papers (throughout the specification and claims “papers” includes virgin or recycled paper, kraft stock and similar materials) find particular application in the container making art wherein such improved properties are desirable. The container making art, particularly, in the field of corrugated containers, folding cartons, and the tray and box industries, consumes much of the natural timber resources. Thus, it would be beneficial to formulate new processes of forming papers of improved wet strength having grease and water resistance properties as well as increase tensile strength which papers would be repulpable and therefore recyclable.
2. Description of the Related Art
The art of “papermaking” is an ancient one, being attributable to invention by the Chinese before the birth of Christ.
As far back as containers have been needed, the use of wood has been the most popular, and has the longest history. Containers in the shape of barrels and crates have traditionally been used to carry and/or store many varied types of materials, including wet products such as produce, fish, meat, and poultry. This of course is not the limit to the requirements of packing wet or refrigerated products as there are many more wet packed products that contain water and ice or condensation from refrigeration to retard the ripening process or to maintain product freshness for distribution over wide geographical areas.
In order to reduce costs, wooden crates were reused as many times as possible. For some products this caused health issues, because bacteria often grow on the surface of wood or in the cracks of the wood. As a result, crossover contamination of bacteria or viruses, such as salmonella, was common, from one crate to another, as proper sanitation was often not performed.
The use of corrugated paper began to mature in the 1930's and 1940's as the container of choice for lightweight items. As the technology increased and the ability to make corrugated boxes out of heavier or thicker paper (or liner), the strength of the corrugated box increased. The corrugation strength of paper was demonstrating strengths that the wood crate manufacturers did not expect. The confidence of the corrugated suppliers along with the innovative minds in the corrugated industry caused a new concept to be considered to perhaps penetrate the wet container market against the wooden crate. This was the introduction of the wax coated corrugated box. If the corrugated box coated with wax could be designed to hold products safely and in vertical stacking stresses that exceed 250 lbs., perhaps the wax would keep the paper/liner dry which would in turn keep the box rigidity and strength as high as in the dry environment, and thus replace the wooden crate. However, in order to increase the strength of a conventional corrugated box, it became necessary to use heavier and thicker paper.
As a result of the superior properties of corrugated paper containers, wood crates were slowly phased out. The wooden crate was pushed out of every market in which the corrugated paper box was suitable for use. Since the 1940's, the wax coated box has done an excellent job of supplying boxes for storing items such as produce, fish, meat and poultry.
More modem developments resulted in the widely accepted Fourdrinier process (See generally Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, pp. 846-7, John Wiley & Sons, New York 1980, herein incorporated by reference in its entirety), in which a “furnish” (a “furnish” is predominantly water, e.g., 99.5% by weight and 0.5% “stock”, i.e., virgin, recycled or mixed virgin and recycled pulp of wood fibers, fillers, sizing and/or dyes) is deposited from a headbox on a “wire” (a fast-moving foraminous conveyor belt or screen) which serves as a table to form the paper. As the furnish moves along, gravity and suction boxes under the wire draw the water out. The volume and density of the material and the speed at which it flows onto the wire determine the paper's final weight.
Typically, after the paper leaves this “wet end” of the papermaking machine, it still contains a predominant amount of water. Therefore, the paper enters a press section, generally comprising a series of heavy rotating cylinders, which press the water from the paper, further compacting it and reducing its water content, typically to 70% by weight.
Thereafter, the paper enters a drying section. Typically, the drying section is the longest part of the paper machine. For example, hot air or steam heated cylinders may contact both sides of the paper, evaporating the water to a relatively low level, e.g., no greater than 10%, typically 2-8% and preferably 5% by weight of the paper.
Following the drying section, the paper optionally passes through a sizing liquid to make it less porous and to help printing inks remain on the surface instead of penetrating the paper. The paper can go through additional dryers that evaporate any liquid in the sizing and coating. Calenders or polished steel rolls make the paper even smoother and more compact. While most calenders add gloss, some calenders are used to create a dull or matte finish.
The paper is wound onto a “parent” reel and taken off the paper making machine.
The paper on the parent reel can be further processed, such as on a slitter/winder, into rolls of smaller size or fed into sheeters, such as folio or cut-size sheeters, for printing end uses or even office application.
In order to make conventional containers, rolls formed by slitter/winder (e.g., of paper and kraft grades of liner) are unwound and coated with a wax. Waxes are used to impart water resistance and wet strength to the liner, but prohibits or otherwise inhibits recycling the used containers incorporating them. Additionally, conventional wax coated liners must be adhered to the other components of the container with hot melt adhesives. Most hot melt adhesives are a further impediment to recycling of formed containers because they employ wax containing components. Thus, there still exists a need for manufacturing paper possessing superior wet and tensile strength and water and grease resistance properties, but facilitating repulping and recycling thereof.
Two methods for coating boxes or other paper products with liquid additives, such as wax, are conventionally used. The first is identified as a curtain coating process. This design incorporates a medium that is impregnated with hot wax and then becomes a corrugated box. A completed, i.e., combined, board is passed through a curtain of hot wax, in a procedure commonly known in the art of paper making as “curtain coating.” First one side and then the opposite side are coated with hot wax. However, due to the conditions necessary to perform the curtain coating process, fire becomes a significant risk.
Another conventional paper coating process is “cascading” The cascading wax procedure is different from the curtain coating procedure in that a regularly corrugated box of any shape or size can be stood on end, such that the corrugated flutes are vertical, to allow the hot wax to permeate the entire structure, with a wax cascading around and through the container in a flat position that is easy to stack for shipping. In contrast to the curtain coating process, the cascading process requires the box to be fully formed prior to application of the wax or other liquid additive. This is considered the better performing wax box of the two described.
Alternative coating procedures are also known in the art, such as those described in U.S. Pat. Nos. 5,858,173; 5,531,863; 5,429,294; and 5,393,566, each of which is herein incorporated by reference in its entirety, for example, surface coating to protect the outside of the liner on both sides to mimic a box subjected to the curtain coating procedure.
Moreover, substitutes for wax coatings have been developed. For example, U.S. Pat. No. 5,393,566 discusses the use of acrylic on the paper machine to generate a moisture barrier. Even with the coated one side liner with the medium included in the design, the acrylic-coated boxes, described therein, equaled the performance of conventional wax coated boxes, coated via the cascade method.
End users of conventional wax boxes are often faced with exorbitant charges for disposal fees, which can often exceed $80/ton of box waste. Because the coatings of the invention may be applied at any existing paper mill, such costs can be reduced to a one time sale of $70/ton, for a total cost savings is $150/ton at current pricing which is significant to national grocers. This industry is what is driving the demand for a solution to the waxed container that has given reliable service for about 60 years.